The present invention relates to an addition tomographic image producing method and an X-ray CT apparatus, and more particularly to an addition tomographic image producing method and an X-ray CT apparatus which, when an addition tomographic image is produced by adding a plurality of tomographic images, can produce an addition tomographic image not adversely affected by the partial volume effect even when the slice thicknesses of the tomographic images are different.
1. First Example (Prior Art)
FIG. 1 is a flow chart showing a first example of a conventional addition tomographic image producing process in a conventional X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows.
In Step ST61, an image index counter j is initialized to xe2x80x9c1xe2x80x9d.
In Step ST62, row data Ij is read from a j-th detector row in the multi X-ray detector.
In Step ST63, an offset correction is performed. Specifically, an offset is subtracted from the row data Ij of the j-th detector row. For brevity of notation, the row data from which the offset has been subtracted will also be designated as
In Step ST64, a detector sensitivity correction is performed. Specifically, the row data Ij of the j-th detector row is divided by an entering X-ray dose Tjxc2x7Io, wherein Tj is the slice thickness of the j-th detector row, and Io is the entering X-ray dose per unit thickness.
In Step ST65, a first correction is applied to the row data Ij/(Tjxc2x7Io) subjected to the detector sensitivity correction. The first correction is a reference correction. For brevity of notation, the row data subjected to the first correction will also be designated as Ij/(Tjxc2x7Io).
In Step ST66, if the value of a certain channel in the row data Ij/(Tjxc2x7Io) subjected to the first correction is not more than a predefined threshold, then the value is replaced by the average value of one or more adjacent channels. This processing will be referred to as Log filtering hereinbelow.
In Step ST67, logarithm processing is performed. Specifically, xe2x88x92Log{Ij/(Tjxc2x7Io)} is calculated.
In Step ST68, a second correction is applied to the row data subjected to the logarithm processing. The second correction is at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction.
In Step ST69, filtering and a back projection operation are applied to the row data subjected to the second correction to obtain a CT image Gj.
In Steps ST70 and ST71, Steps ST62-ST69 are repeated for j=2-J. Thus, CT images GI-GJ are reconstructed.
In Step ST72, the CT images GI-GJ are simply added to obtain an addition CT image G.
2. Second Example (Prior Art)
FIG. 2 is a flow chart showing a second example of a conventional addition tomographic image producing process in a conventional X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows.
In Step ST81, an image index counter j is initialized to xe2x80x9c1xe2x80x9d.
In Step ST82, row data Ij is read from a j-th detector row in the multi X-ray detector.
In Step ST83, an offset correction is performed. Specifically, an offset is subtracted from the row data Ij of the j-th detector row. For brevity of notation, the row data from which the offset has been subtracted will also be designated as Ij.
In Step ST84, a detector sensitivity correction is performed. Specifically, the row data Ij of the j-th detector row is divided by an entering X-ray dose Tjxc2x7Io.
In Step ST85, a first correction is applied to the row data Ij/(Tjxc2x7Io) subjected to the detector sensitivity correction. The first correction is a reference correction. For brevity of notation, the row data subjected to the first correction will also be designated as Ij/(Tjxc2x7Io).
In Step ST86, Log filtering is performed.
In Step ST87, logarithm processing is performed. Specifically, xe2x88x92Log{Ij/(Tjxc2x7Io)} is calculated.
In Step ST88, a second correction is applied to the row data subjected to the logarithm processing. The second correction is at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction.
In Steps ST89 and ST90, Steps ST82-ST88 are repeated for j=2-J. Thus, row data Ixe2x80x21-Ixe2x80x2J subjected to the second correction are obtained.
In Step ST91, the row data Ixe2x80x21-Ixe2x80x2J subjected to the second correction are simply added to obtain addition data I, that is:
I=xcexa3Ixe2x80x2j
In Step ST92, filtering and a back projection operation are applied to the addition data I to obtain an addition CT image G.
In the first and second examples of the conventional technique, when the slice thicknesses of the detector rows are equal, there is no significant problem in obtaining the addition CT image G.
However, when the slice thicknesses are different, an adverse effect of the partial volume appears on the addition CT image G. Specifically, the different slice thicknesses produce different partial volume effects, resulting in different CT values for the same substance. In other words, discontinuity arises in the CT values of the same substance. Accordingly, the CT values of an object which should appear to be uniform appear to partially vary on the addition CT image G.
It is therefore an object of the present invention to provide an addition tomographic image producing method and an X-ray CT apparatus which, when an addition tomographic image is produced by adding a plurality of tomographic images, can produce an addition tomographic image not adversely affected by the partial volume effect even when the slice thicknesses of the tomographic images are different.
In accordance with a first aspect, the present invention provides an addition tomographic image producing method comprising the steps of: applying an offset correction and a sensitivity correction to data Ij (j=1-J) from which respective tomographic images Gj can be reconstructed; generating addition data by performing a weighted addition on the data Ij with a weight Tj/xcexa3Tj corresponding to the slice thickness Tj; applying logarithm processing to the addition data; and performing reconstruction on the addition data to produce an addition tomographic image G.
According to the addition tomographic image producing method in the first aspect, the addition data is generated by performing a weighted addition on the data Kj subjected to the sensitivity correction with a weight Tj/xcexa3Tj corresponding to the slice thickness Tj. Specifically, when the addition data is represented as K,
K=xcexa3{(Tj/xcexa3Tj)xc2x7Kj
As explained further later, K is equivalent to sensitivity-corrected data of one slice having a slice thickness of xcexa3Tj. Therefore, no adverse effect of the partial volume will appear on the addition tomographic image G produced by adding a plurality of tomographic images Gj having different slice thicknesses.
In accordance with a second aspect, the present invention provides the addition tomographic image producing method of the aforementioned configuration, wherein the tomographic image Gj is an X-ray CT image, and the data Ij is row data.
According to the addition tomographic image producing method in the second aspect, the addition data K is generated by performing a weighted addition on the row data Kj subjected to the sensitivity correction with a weight Tj/xcexa3Tj corresponding to the slice thickness Tj. Specifically,
K=xcexa3{(Tj/xcexa3Tj)xc2x7Kj
As described later, K is equivalent to sensitivity-corrected row data of one slice having a thickness of xcexa3Tj . Therefore, no adverse effect of the partial volume will appear on the addition tomographic image G produced by adding a plurality of X-ray CT images Gj having different slice thicknesses.
In accordance with a third aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, wherein the row data is that obtained by each detector row in a multi X-ray detector.
According to the addition tomographic image producing method in the third aspect, a tomographic image can be produced from row data obtained by detector rows in a multi X-ray detector as if it were reconstructed from row data obtained by one detector row which is a combination of those detector rows.
In accordance with a fourth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of applying a reference correction to the row data before or after the sensitivity correction, or to the addition data before the logarithm processing
According to the addition tomographic image producing method in the fourth aspect, when applied to the row data before or after the sensitivity correction, the reference correction may be performed in an ordinary way. When applied to the addition data before the logarithm processing, the reference correction needs to be performed only once.
In accordance with a fifth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of applying at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction to the addition data subjected to the logarithm processing.
According to the addition tomographic image producing method in the fifth aspect, since the several corrections are applied to the addition data, the corrections need to be performed only once.
In accordance with a sixth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of, when a value of a certain channel in the row data before the logarithm processing is not more than a predefined threshold, replacing the value by an average value of one or more adjacent channels.
According to the addition tomographic image producing method in the sixth aspect, when an abnormal value is contained in the row data, the value can be removed before the logarithm processing.
In accordance with a seventh aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, wherein the step of performing reconstruction comprises filtering and a back projection operation.
According to the addition tomographic image producing method in the seventh aspect, a tomographic image can be reconstructed by a filtered back projection technique.
In accordance with a eighth aspect, the present invention provides an addition tomographic image producing method comprising the steps of: applying an offset correction to data Ij (j=1-J) from which respective tomographic images Gj can be reconstructed; generating addition data I by simply adding the data Ij; applying a sensitivity correction and logarithm processing to the addition data I; and performing reconstruction on the addition data to produce an addition tomographic image G.
According to the addition tomographic image producing method in the eighth aspect, the addition data I is generated by simply adding the data Ij subjected to only the offset correction and before they are subjected to the sensitivity correction. Specifically,
xe2x80x83I=xcexa3Ij.
I is equivalent to data of one slice having a slice thickness of xcexa3Tj. Therefore, no adverse effect of the partial volume will appear on the addition image G produced by adding a plurality of tomographic images Gj having different slice thicknesses.
Moreover, since the offset correction is applied to the data Ij (j=1-J), the processing may be the same as the ordinary one.
In accordance with a ninth aspect, the present invention provides an addition tomographic image producing method comprising the steps of: generating addition data I by simply adding data Ij (j=1-J) from which respective tomographic images Gj can be reconstructed; applying an offset correction, a sensitivity correction and logarithm processing to the addition data I; and performing reconstruction on the addition data to produce an addition tomographic image G.
According to the addition tomographic image producing method in the ninth aspect, the addition data I is generated by simply adding the data Ij before they are subjected to the offset correction and the sensitivity correction. Specifically,
I=xcexa3Ij.
I is equivalent to data of one slice having a slice thickness of xcexa3Tj. Therefore, no adverse effect of the partial volume will appear on the addition tomographic image G produced by adding a plurality of tomographic images Gj having different slice thicknesses.
Moreover, since the offset correction is applied to the addition data I, the offset correction needs to be performed only once, although offset values for data Ij need to be added beforehand.
In accordance with a tenth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, wherein the data Ij is row data obtained by each detector row in a multi X-ray detector.
According to the addition tomographic image producing method in the tenth aspect, a tomographic image can be produced from row data obtained by detector rows in a multi X-ray detector as if it were reconstructed from row data obtained by one detector row which is a combination of those detector rows.
In accordance with a eleventh aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of applying a reference correction to the addition data I before the sensitivity correction, or after the sensitivity correction and before the logarithm processing.
According to the addition tomographic image producing method in the eleventh aspect, when applied to the row data before or after the sensitivity correction, the reference correction may be performed in an ordinary way. When applied to the addition data I before the logarithm processing, the reference correction needs to be performed only once.
In accordance with a twelfth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of applying at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction to the addition data subjected to the logarithm processing.
According to the addition tomographic image producing method in the twelfth aspect, since the several corrections are applied to the addition data I, the corrections need to be performed only once.
In accordance with a thirteenth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, comprising the step of, when a value of a certain channel in the addition data before the logarithm processing is not more than a predefined threshold, replacing the value by an average value of one or more adjacent channels.
According to the addition tomographic image producing method in the thirteenth aspect, when an abnormal value is contained in the row data, the value can be removed before the logarithm processing.
In accordance with a fourteenth aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, wherein the step of performing reconstruction comprises filtering and a back projection operation.
According to the addition tomographic image producing method in the fourteenth aspect, a tomographic image can be reconstructed by a filtered back projection technique.
In accordance with a fifteenth aspect, the present invention provides an X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows, comprising: sensitivity correction means for applying an offset correction and a sensitivity correction to row data Ij (j=1-J) obtained by the detector rows; addition data generating means for generating addition data I by performing a weighted addition on the row data subjected to the sensitivity correction with a weight Tj/xcexa3Tj corresponding to the slice thickness Tj; logarithm processing means for applying logarithm processing to the addition data I; and reconstruction means for applying filtering and a back projection operation to the addition data subjected to the logarithm processing.
According to the X-ray CT apparatus in the fifteenth aspect, the addition tomographic image producing method of the second aspect can be suitably practiced.
In accordance with a sixteenth aspect, the present invention provides the X-ray CT apparatus of the aforementioned configuration, comprising first correcting means for applying a reference correction to the row data before or after the sensitivity correction, or to the addition data I before the logarithm processing.
According to the X-ray CT apparatus in the sixteenth aspect, the addition tomographic image producing method of the fourth aspect can be suitably practiced.
In accordance with a seventeenth aspect, the present invention provides the X-ray CT apparatus of the aforementioned configuration, comprising second correcting means for applying at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction to the addition data subjected to the logarithm processing.
According to the X-ray CT apparatus in the seventeenth aspect, the addition tomographic image producing method of the fifth aspect can be suitably practiced.
In accordance with an eighteenth aspect, the present invention provides an X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows, comprising: offset correcting means for applying an offset correction to row data Ij (j=1-J) obtained by the detector rows; addition data generating means for generating addition data I by simply adding the row data subjected to the offset processing; sensitivity correcting means for applying a sensitivity correction to the addition data I; logarithm processing means for applying logarithm processing to the addition data subjected to the sensitivity correction; and reconstruction means for applying filtering and a back projection operation to the addition data subjected to the logarithm processing.
According to the X-ray CT apparatus in the eighteenth aspect, the addition tomographic image producing method of the eighth aspect can be suitably practiced.
In accordance with a nineteenth aspect, the present invention provides an X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows, comprising: addition data generating means for generating addition data I by simply adding row data Ij (j=1-J) obtained by the detector rows; offset correcting means for applying an offset correction to the addition data I; sensitivity correcting means for applying a sensitivity correction to the addition data subjected to the offset processing; logarithm processing means for applying logarithm processing to the addition data subjected to the sensitivity correction; and reconstruction means for applying filtering and a back projection operation to the addition data subjected to the logarithm processing.
According to the X-ray CT apparatus in the nineteenth aspect, the addition tomographic image producing method of the ninth aspect can be suitably practiced.
In accordance with a twentieth aspect, the present invention provides the X-ray CT apparatus of the aforementioned configuration, comprising first correcting means for applying a reference correction to the addition data before the sensitivity correction, or after the sensitivity correction and before the logarithm processing.
According to the X-ray CT apparatus in the twentieth aspect, the addition tomographic image producing method of the eleventh aspect can be suitably practiced.
In accordance with a twenty-first aspect, the present invention provides the X-ray CT apparatus of the aforementioned configuration, comprising second correcting means for applying at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction to the addition data subjected to the logarithm processing.
According to the X-ray CT apparatus in the twenty-first aspect, the addition tomographic image producing method of the twelfth aspect can be suitably practiced.
Thus, according to the addition tomographic image producing method and the X-ray CT apparatus of the present invention, when an addition tomographic image is produced by adding a plurality of tomographic images, an addition tomographic image can be produced which is not adversely affected by the partial volume effect even when the slice thicknesses of the tomographic images are different.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.